Return of volunteers to continue after much delay (2,4)
I believe the answer is:
at last
'after much delay' is the definition.
(I've seen this before)
'return of volunteers to continue' is the wordplay.
'return of' says the letters should be written backwards.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'to continue' becomes 'last' (I've seen this before).
'ta' backwards is 'at'.
'at'+'last'='AT LAST'
(Other definitions for at last that I've seen before include "Where cobbler located" , "Finally, after a long wait" , "Finally, having waited a long time" , "Where a snob worked" , "In the end, after much delay" .)